The present invention relates to an objective lens drive device for driving an objective lens perpendicularly to the optical axis and a data track, in order to correct relative misalignment between the data track and a light spot which is projected by the objective lens, that is, to correct the tracking error, in a device for reading out data by projecting a readout light spot on the data track in which data is recorded spirally or coaxially on a recording medium.
The data readout device of the type described above is conventionally known. For example, a recording medium which has data tracks is called a video disk. The video disk in which the encoded video and audio signals are recorded as optical data such as optical transmission, reflection, and phase characteristics, is rotated at a high speed while a laser beam radiated from a laser source is focused on the video disk. Thus, the optical data is read out. One of the features of this recording medium is the high recording density of the data, so that the respective tracks are very narrow and the spaces between the tracks are also narrow. Accordingly, the diameter of the readout light spot is extremely small. In order to correctly read out original data on data tracks whose track and pitch are narrow, relative misalignment between the readout light spot and the data track, that is, tracking error, must be reduced to the minimum. Conventionally, for this purpose, the relative misalignment between the readout light spot and the data track is detected. Based on the detected tracking error signal, the readout light spot is displaced along the width of the data track to achieve the desired tracking control. For example, a tracking control mechanism is proposed in which a vibration mirror is disposed in the light path between the laser source and the objective lens, and the mirror is rotated by a tracking error signal. However, with this tracking mechanism, it is difficult to obtain a satisfactorily precise device which is compact in size and low in cost.
For eliminating the drawbacks described above, a method is proposed in which an elastic supporting member comprising a leaf spring supports the objective lens or its holding frame and the objective lens is displaced perpendicularly to the optical axis and the data track, according to the tracking error signal. In a drive device using this leaf spring, a system utilizing an electromagnet, a voice coil system or a system utilizing a piezoelectric element is proposed to displace the objective lens. However, the drive device must be small in size and light in weight in order to maintain excellent response characteristics of the tracking. Further, in practice, a focusing error resulting from incorrect focusing of the light spot on the data track, in addition to the tracking error, occurs in the device. In order to correct this focusing error, a focusing mechanism which displaces the objective lens in the direction of the optical axis is also required. When the tracking mechanism is mounted on the focusing mechanism, the tracking mechanism must be small in size and light in weight for performing excellent focusing correction. When a device utilizing an electromagnet is used, the necessary driving force can be obtained and the device is relatively easily made compact and light. However, the relation between the current flowing through the electromagnetic coil and the displacement of the objective lens becomes nonlinear, making simple correction of the tracking error impossible. In the case of a device with a voice coil system, the device cannot be made compact and light in weight. When a device utilizing a piezoelectric element is used, the necessary driving force cannot be obtained.
In order to eliminate the drawbacks described above, the present applicant proposed an objective lens drive device in U.S. patent application Ser. No. 217,117 filed on Dec. 17, 1980, now U.S. Pat. No. 4,386,823, issued June 7, 1983. In this device, the necessary driving force drives the objective lens linearly by the tracking error signal. In particular, the device performs two-dimensional driving of the objective lens and is small in size and light in weight.
FIGS. 1A and 1B show arrangements and principles of operation of drive means of objective lens drive devices. In an example shown in FIG. 1A, first and second stationary yokes 2 and 2' are connected to a permanent magnet 1, and a magnetic circuit shown by arrow A is formed. A movable member 5 constituted by a magnetic body is movably disposed in the direction shown by arrow B in a gap between the first and second yokes 2 and 2'. First and second coils 4 and 4' are wound around the first and second yokes 2 and 2', respectively. If leakage of the magnetic flux is not considered, only the opposing surfaces 4A and 4A' of the first and second coils 4 and 4' are in the magnetic field which is formed by the permanent magnet 1. In other words, if it is assumed that the cross section of the yokes 2 and 2' is square, the three respective surfaces of the yokes 2 and 2' other than the opposing surfaces are not crossed by lines of the magnetic flux. In this way, when a current flows through a coil in the magnetic field, the coil is subjected to a force. When a current flows through the first and second coils 4 and 4' so that the coil portions 4A and 4A' are urged in the same direction (current flowing as shown in the symbol in FIG. 1A), the coils 4 and 4' do not move since they are wound around the stationary yokes. The movable magnetic body 5 moves under the force in one of the directions shown by the arrow B; for example, to the left. When the current flows through the coils 4 and 4' in the reverse direction, the movable magnetic body 5 moves in the opposite direction, that is, to the right.
Referring to FIG. 1B, the first and second stationary yokes 2 and 2' are connected to a third stationary yoke 3 in place of the permanent magnet. A movable member 6 constituted by a permanent magnet is disposed in a gap between the first and second yokes 2 and 2' and moves in the direction shown by the arrow B in order to generate the magnetic flux through the yokes. A magnetic circuit shown by the arrow A is thus formed. As in the case described above, when a current flows through the first and second coils 4 and 4' which are wound around the first and second stationary yokes 2 and 2', the movable member 6 can be displaced in the direction shown by the arrow B. The first, second and third stationary yokes 2, 2' and 3 may be integrally formed, or the third yoke 3 may be replaced with a permanent magnet as shown in FIG. 1A.
FIGS. 2A and 2B and FIGS. 3A and 3B show arrangements of an objective lens drive device in which pairs of the respective drive means shown in FIGS. 1A and 1B oppose each other so that the magnetic flux distribution becomes symmetrical about a plane defined by the optical axis of an objective lens and longitudinal data tracks. An objective lens 7 is mounted to a lens holding frame 8 which is constituted by a nonmagnetic body, and movable members 5-1 and 5-2 constituted by magnetic bodies (FIGS. 2A and 2B) and movable members 6-1 and 6-2 constituted by permanent magnets (FIGS. 3A and 3B) are respectively attached to the holding frame 8 so as to make the magnetic flux distribution symmetrical about the plane defined by the optical axis of the objective lens 7 and the longitudinal data tracks. One end of a leaf spring 9 and one end of a leaf spring 9' which is paired therewith are disposed symmetrically about the plane described above and are mounted to the movable members 5-1, 5-2, 6-1 and 6-2. The other ends of these leaf springs are mounted respectively to stationary members 10 and 10' and support the objective lens or the like so that the objective lens or the like is movable in the direction shown by the arrow B. Two pairs of the drive means are opposed to each other and cooperate with the movable members 5-1, 5-2, 6-1 and 6-2. Respective arrangements of the paired drive means are the same as in FIGS. 1A and 1B. The stationary yokes 2, 2' and 3 are integrally formed in FIGS. 3A and 3B.
When a current flows through the first and second coils 4 and 4' in the direction as shown in FIGS. 1A and 1B, a magnetic field as shown by arrow C is induced in the first and second stationary yokes 2 and 2'. The direction of the induced magnetic field C is the same as that of a magnet field generated by the permanent magnets 1 and 6, so that the total amount of magnetic flux increases. When the current flowing through the coils 4 and 4' is reversed, the magnetic field is induced in the direction opposite to the direction shown by the arrow C, thus reducing the total amount of the magnetic flux. Therefore, the relation between the current flowing through the coils and the displacement of the movable members 5 and 6 becomes nonlinear.
On the other hand, when a current flows through coils 4-1, 4-1', 4-2, 4-2' in the direction shown in FIGS. 2A and 2B and FIGS. 3A and 3B, the movable members 5-1, 5-2, 6-1 and 6-2 are displaced in the direction of the arrow B. However, when the magnetic field (shown by the arrow C) induced at a first pair of stationary yokes 2-1 and 2-1' is parallel to the magnetic field of permanent magnets 1-1 and 6-1, the magnetic field induced at the second pair of stationary yokes 2-2 and 2-2' opposes the magnetic field (shown by arrow A-2) of permanent magnets 1-2 and 6-2. In this way, the first pair of stationary yokes and the second pair of stationary yokes compensate for each other, so that the current flowing through the coils and the displacement of the movable members becomes linear, and a great force is obtained to drive the movable members.
FIGS. 4A and 4B show one arrangement of the objective lens drive device in which the objective lens 7 is two-dimensionally displaced in the direction of the tracking error correction described above and in the direction of the optical axis of the objective lens, that is, in the direction of the focusing correction. A drive mechanism for displacing the objective lens 7 in the direction of the tracking error correction may be one of the arrangements shown in FIGS. 2A and 2B and FIGS. 3A and 3B. In this embodiment, it is the arrangement shown in FIGS. 2A and 2B.
The movable members 5-1 and 5-2 having the objective lens 7, the lens holding frame 8, and a magnetic body are movably mounted to an inner frame 11 only in the direction shown by the arrow B through a pair of leaf springs 9 and 9' which are disposed symmetrically about the optical axis of the objective lens 7. This inner frame 11 (integrally formed with an inner frame 11') is mounted to an outer frame 13 by a pair of circular springs 12 and 12' with spiral parts. Therefore, the inner frame 11 can be displaced in the direction of the optical axis of the objective lens 7. The stationary yokes 2-1, 2-1', 2-2 and 2-2' and the permanent magnets 1-1 and 1-2 which cooperate with the movable members 5-1 and 5-2 constituted by the magnetic bodies are held in the outer frame 13. The objective lens 7, the lens holding frame 8 and the movable members 5-1 and 5-2 are only mounted through the leaf springs 9 and 9' to the inner frame 11 which is movable in the direction of the optical axis, resulting in a lightweight construction. When moved in the direction of the optical axis, the stationary yokes 2-1, 2-1', 2-2, and 2-2' are displaced more than the movable members 5-1 and 5-2 in the optical axis direction, so the magnetic flux crossing the movable members 5-1 and 5-2 stays constant. In order to perform focusing by displacing the inner frames 11 and 11' in the direction of the optical axis, a coil 15 for focusing is wound around a bobbin 14 which is integrally formed with the inner frames 11 and 11'. A permanent magnetic 16, and stationary yokes 17 and 18 which cooperate with the coil are mounted to the outer frame 13.
A magnetic circuit formed by a focusing drive means as in FIGS. 4A and 4B is coaxially symmetrical about the optical axis as is apparent in the figures. Therefore, the distribution of the magnetic flux which leaks from the magnetic circuit becomes coaxially symmetrical about the optical axis. Since a frequency component which is the same as the frequency by which the recording medium rotates is mostly included in the focusing error signal component and the tracking error signal component, the respective resonant frequencies of the focusing drive means and the tracking drive means are generally set to correspond with the rotational frequency of the recording medium.
The mass (about 3 grams) of the movable portion of the focusing mechanism is greater than the mass (about 0.5 gram) of the movable portion of the tracking mechanism, so that the magnetic flux generated by the permanent magnet 16 is sufficiently greater than the magnetic flux generated by the permanent magnets 1-1 and 1-2. Therefore, as for the magnetic flux leakage, the influence of the focusing mechanism on the tracking mechanism is greater than the influence of the tracking mechanism on the focusing mechanism. The magnetic flux leakage of the focusing mechanism is coaxially symmetrical about the optical axis. On the other hand, the distribution of the magnetic flux of the tracking mechanism is symmetrical about the plane defined by the optical axis of the objective lens 7 and the longitudinal data tracks. However, the leakage of the magnetic flux from the focusing mechanism influences the tracking mechanism. The distribution of the magnetic flux of the tracking mechanism becomes asymmetrical about the plane defined by the optical axis of the objective lens 7 and the direction shown by the arrow B (For example, if the magnetic flux which passes through the stationary yokes 2-1 and 2-2 increases, the magnetic flux passing through the yokes 2-1' and 2-2' decreases).
Under this condition, assume that the objective lens 7, the lens holding frame 8, the springs 9 and 9', the inner frames 11 and 11', the bobbin 14 and the coil 15 are displaced in the focusing direction. Since these movable members are held at the outer frame 13 through the circular springs 12 and 12' with spiral parts, they are displaced from the balanced position to the direction of the optical axis of the objective lens 7 while rotating about the optical axis in direction D shown in the figure. The force is not only exerted on the movable members 5-1 and 5-2 in the direction shown by the arrow B, but it also attracts the movable members 5-1 and 5-2 to the stationary yokes 2-1, 2-1', 2-2 and 2-2'. This attractive force is not linear with respect to the distance between the movable members 5-1 and 5-2 and the stationary yokes 2-1, 2-1', 2-2 and 2-2'. The narrower the gap the greater the force exerted. The pair of leaf springs 9 and 9' which hang the movable members 5-1 and 5-2 has considerable resistance to the attractive force in this direction. However, the springs show weak resistance in the direction of torsion displacement. Therefore, when the gap distance becomes short by rotating the movable members 5-1 and 5-2 about the optical axis, the springs are attracted when the attractive force is greater than the restoring force against the torsion displacement of the leaf springs 9 and 9'. Further, since the attractive force of the stationary yokes for the plane defined by the optical axis of the objective lens 7 and the direction shown by the arrow B under the influence of the leaked magnetic flux from the focusing mechanism varies, the attractive action is increased. Therefore, correct tracking control and focusing control cannot be performed.